Early mobile communication devices used dedicated channels for voice communication. Voice communication, at that point in time, constituted the majority of all data transmitted and received through mobile devices. As mobile technology advanced, alternate forms of data were introduced to mobile devices, including text messages and emails, and eventually broadband web access. Dedicated voice channels persisted through most of this progression until broadband web access became the dominant consumer of bandwidth for mobile devices.
Many modern mobile communication devices, particularly cellphones, smartphones and tablet computers, are shifting to IMS based voice communication rather than dedicated voice channels. This shifting is driven by the ever expanding use of broadband web access to stream audio and video content. This expansion has made the volume of data required for voice communication an inconsequential portion of all data transmitted and received by a given device. Additionally, the amount of available bandwidth has dwarfed the demands of voice communication.
A voice-only or video/voice call can be carried out over an IMS session. An IMS session is an IP connection over which IMS packets are transmitted and received. The IMS packets are simply transmitted and received along with all other IP packets for the device. An enabling factor of the shift to IMS is the ability of networks to efficiently route IMS packets from a source to a destination. The efficiency of a network is largely driven by the latency introduced and the level of accuracy of transmissions. Other factors, such as power, contribute to the efficiency of a network, but the telecommunication industry often focuses on time and accuracy.
One development that has furthered IMS communication is the availability of packet-switched (PS) networks over alternate, circuit-switched (CS) networks. In a CS network, data is routed over a dedicated circuit from point-to-point. No other network traffic can use the dedicated circuit as long as the connection exists. In a PS network, each message is broken into packets that are routed point-to-point. Each packet seeks out the most efficient route to reach its destination. The message is then reassembled at the receiving end.
CS networks are most common in voice applications, where terminals are relatively limited and the volume of data is relatively low. Outside of that, when the volume of data becomes large and the access points grow exponentially, as in broadband web access, it becomes more efficient to use PS networks, where the network load can be better distributed.
Modern mobile networks provide network bandwidth primarily through PS networks, although wider coverage is available on some CS networks. Consequently, mobile devices tend to rely mostly on the PS networks for carrying out IMS sessions, and fall back on the CS networks as needed. It is often the case that an IMS session originates on a PS network, such as a long-term evolution (LTE) network, and a party's mobile communication device moves from an area with LTE coverage to an area without. In that case, the mobile communication device must hand over the IMS session from the PS network to a CS network, such as a second or third generation (2G/3G) network. PS network coverage being as limited as it is, the telecommunication industry contemplated the scenario and established procedures for carrying out the handover. These procedures are known as the single radio voice call continuity (SRVCC) procedure and the video SRVCC (vSRVCC) procedure. The SRVCC procedure is for handing over a voice IMS session, while the vSRVCC procedure is for handing over a video/voice IMS session. The SRVCC procedure and the vSRVCC procedure are established by the Third Generation Partnership Program (3GPP), which maintains the 3GPP standard that applies to most modern mobile communication networks and devices.